Motor heater unit

ABSTRACT

A motor heater unit is disclosed herein for maintaining an AC electric motor in a warm condition when the motor is turned off. The unit comprises in combination a main power source, a plurality of connector means connecting said main power source to said motor, at least all but one of said plurality of connector means containing switch means for breaking the connection between the motor and the main power source, a timing circuit adjusted to pass a portion of the wave form of an electrical signal applied thereto, the timing circuit being connected across the first of said switch means, a gate means connected to receive the output of said timing circuit, an auxiliary source of electrical energy connected across said first switch means and further connected to receive the output of said gate means, said auxiliary source of electrical energy providing electrical energy only when said gate means triggers said auxiliary source of electrical energy, and, a back e.m.f. quenching means connected across said first switch means.

United States Patent 1 Hann [ MOTOR HEATER UNIT [76] Inventor: Donald R.Hann, 13501 Ward Way,

Saratoga, Calif. 95070 [22] Filed: Sept. 17, 1971 [21] Appl. No.:181,497

[52] US. Cl. 318/436, 318/473 [51] Int. Cl. H02b 1/02 [58] Field ofSearch 318/436, 558, 473,

[ Nov. 20, 1973 [5 7 ABSTRACT A motor heater unit is disclosed hereinfor maintain ing an AC electric motor in a warm condition when the motoris turned off. The unit comprises in combination a main power source, aplurality of connector means connecting said main power source to saidmotor, at least all but one of said plurality of connector meanscontaining switch means for breaking the connection between the motorand the main power source, a timing circuit adjusted to pass a portionof the wave form of an electrical signal applied thereto, the timingcircuit being connected across the first of [56] References Cited UNITEDSTATES PATENTS said switch means, a gate means connected to receive theoutput of said timing circuit, an auxiliary source of electrical energyconnected across said first switch 2240207 4/1941 Crawford 318/558 meansand further connected to receive the output of 3:403:315 9/1968 lx:318/227 said gate means, said auxiliary source of electrical en-2,338,518 1/1944 K 31g 553 ergy providing electrical energy only whensaid gate 3,445,743 5/1969 Blair 318/436 means triggers said auxiliarysource of electrical en- 3,582,712 6/1971 Blair 318/473 X ergy, and, aback e.m.f. quenching means connected across said first switch means.

Primary Examiner-B. Dobeck Attorney -l-lubert E. Duff 7 h m 4 DrawingFigures 2 MAIN CONNECTOR POWER 4 SOURCE CONNECTOR a MOTOR RESISTANCETIMING CIRCUIT GATE MEANS AUXILIARY SOURCE OF ELECTRICAL ENERGY BACK E MF ABSORBER PAIENIEDNBVEO ms 3; 7 74.096

SHEET 19F 2 CTO 2 MAIN a CONNE R POWER 4 SOURCE W CONNECTOR B 7 MOTORRESISTANCE TIMING CIRCUIT GATE MEANS AUXILIARY SOURCE OF ELECTRICALENERGY BACK EMF ABSORBER K9 F/g I I7 I A O' 5% [72 M OTOR LIE 25 30 3/INVENTOR. DONALD R. HANN 29 F lg 2 PATENTEDHOVZO I975 3.774.096

SHEET 2 BF 2 mo v. +||o v.

4 +30\/ I +30 v. 0v i 7 ov. --30 V -3 v -no v. -||o v.

TIMING CIRCUIT OUTPUT LINE VOLTAGE Fig 3 VOLTAGE OR CURRENT c d 00 b 127W FULL CYCLE OUTPUT OF AUXILIARY SOURCE OF ELECTRICAL ENERGY TRIGGEREDOUTPUT OF AUXILIARY SOURCE OF ELECTRICAL ENERGY Fig 4 INVENTOR. DONALDR. HANN MOTOR HEATER UNIT BACKGROUND OE THE INVENTION This inventionrelates to a motor heater unit for maintaining an AC electric motor in awarm condition after the motor is turned off.

One of the main, if not the main, causes of failure in electrical motorsis the condensation of moisture on motor windings when the motors arenot in operation. When the motor is turned on after moisture hascondensed on the windings a short can occur and the entire motor can bethereby burnt out. This is a well known problem recognized by the art.

Many efforts have been made in the past to eliminate condensation withinthe windings and on the electrical connections of electric motors whenthe motors are not in operation. Radiant or space heaters are utilizedin some instances to prevent such condensation within electric motorswhen the motors are not in operation. The interior portions of the motorwindings and other inaccessable electrical connections are however notalways maintained at the desired elevated temperatures by the use ofradiant or space heaters. Further, it is generally not economicallypracticable to provide radiant or space heaters except for very largemotors such as 200 horsepower (HP) or larger (NEMA size 6 or above);yet, the great majority of motors are below 200 HP in size (NEMA sizesl-).

Another example of prior art motor heater units and processes formaintaining an AC motor in a warm condition when the motor is not inoperation comprises providing a small current, well below the ratedoperating amperage of the motor, fed into the electrical input leads ofthe motor. The small current keeps the interior of the motor at anelevated temperature and thereby prevents condensation therein. For suchunits to operate properly however, it is most desirable that they beprovided with timing circuits which prevent the heating current frombeing supplied until the motor has stopped turning after it has beenswitched ofi. This prevents the unit supplying the heating current frombeing damaged by back e.m.f. induced in the armature of the motor. US.Pat. No. 3,445,743 issued May 20, 1969 to D. J. Blair describes thistype of motor heater unit.

Motor heater units which provide a small current, considerably less thanthe rated operating amperage of the motor, to the input leads of themotor when the motor is not in operation have the followingdisadvantages: (1) For proper operation, the motor heating unit must beactivated on a time-delay basis to protect the motor heating unit frombeing damaged by the back e.m.f. induced in the armature of the motorimmediately after it has been turned off. Alternatively the motorheating unit must not be connected to the motor until the motor hascompletely stopped turning. (2) This type of motor heater unit isgenerally physically quite large and heavy because it requires atransformer and complicated switching circuitry. (3) This type of motorheater unit is generally quite expensive because of the relatively highcost of the components and the cost of assembling these components. (4)The relatively large size and great weight of these motor heatingunitsmakes it difficult, if not impossible, to'hand them withoutsupportupon the motor control box.

In view of the foregoing it is-an object of-the present invention toprovide a small, light-weight an inexpensive motor heater unit which canbe directly connected without support to the motor control box.

It is a further object of the invention to provide a motor heater unitthat does not require time delay circuitry to protect the motor heaterfrom back e.m.f. induced in the armature of the motor immediately afterthe motor is turned ofi'.

It is a still further object of the invention to provide a small andinexpensive motor heater unit which is constantly connected to the motorand does not require a complicated switching arrangement to activate itwhen the motor is not in operation.

It is an additional object of the invention to provide a motor heaterunit satisfying all of the above objects and further satisfying theelectrical code requirements as set out in the National Electric Codeand by the National Electrical Manufacturers Association.

Another object of the invention yet is to provide a novel process formaintaining an AC electric motor in a warmed condition.

Other objects of the invention will become apparent from the descriptionwhich follows herein.

SUMMARY OF THE INVENTION The invention comprises a motor heater unit anda process for maintaining an AC electric motor in a warm condition whenthe motor is not in operation. The motor heater unit comprises incombination a main power source, a plurality of connector meansconnecting said main power source to said motor, at least all but one ofsaid plurality of connector means containing switch means for breakingthe connection between the motor and the main power source, a timingcircuit adjusted to pass a portion of the wave form of an electricalsignal applied thereto and being connected across the first of saidswitch means, a gate means connected to receive the output of saidtiming circuit, an auxiliary source of electrical energy connectedacross said first switch means and further connected to receive theoutput of said gate means, said auxilary source of electrical energyproviding electrical energy only when said gate means triggers saidauxiliary source of electrical energy, and a back e.m.f. quenching meansconnected across said first switch means.

The invention further comprises a process for maintaining an AC electricmotor in a warmed condition when the motor is not in operation bysupplying a chopped alternating current signal thereto.

The motor heater unit described herein and the process described hereinaccomplish the objects set out above.

BRIEF DESCRIPTION OF THE DRAWINGS For a more detailed understanding ofthe motor heater unit of the invention reference may be had to theaccompanying drawings which are hereby incorporated in and made a partof this specification.

FIG. 1 illustrates a block diagram showing the basic heating unit forthe motor and its connections to the motor. FIG. 1 shows the motor as asingle phase motor.

FIG. 2 illustrates a circuit diagram showing the various components inthe heating unit in combination with a three-phase motor.

FIG. 3 illustrates the operation at volts of the timing circuit.

FIG. 4 illustrates the output of the auxiliary power source.

msan w f r e a e GENERAL DESCRIPTION OF THE INVENTION Referring to FIG.1 there is illustrated a Motor, 1 normally of 200 HP or less rating andmore usually of 100 HP or less rating. The motor might be powered forexample by a l 10 volt AC single phase power source. The motor isconnected via Connector 2, Connector 3, and Switch 4 to a terminal board(not shown) which in turn is connected to a main power source.

Connected across the Switch 4 is a Timing Circuit 5. The timing circuitprovides a series of electrical impulses to Gate 6. The timing citcuitis isolated from the Connector 3 by the Resistance 7. The Gate 6, whichis activated only when the voltage applied to it from the timing circuitreaches at least a minimum positive or negative value, e.g., i 30 voltsserves to trigger the Auxiliary Source of Electrical Energy 8. When lessthen the minimum positive or negative voltage is applied to the Gate 6,the voltage is dissipated via the Resistance 7. When the AuxiliarySource of Electrical Energy 8 is triggered it provides a portion of acycle of current, alternately positive or negative, in response to thesignal applied to it from the Gate 6. The portions of a cycle of currentfrom the Auxiliary Source of Electrical Energy 8 are supplied to theMotor 1 and they serve to keep the motor warm. The Back e.m.f. QuenchingCircuit 9 is also connected across Switch 4. This serves to quench anyback e.m.f. induced in the armature of the motor immediately after theSwitch 4 is placed in the open position.

It should be noted that when the switch 4 is in the closed position theentire heating unit is still connected to the motor but the unit iscompletely inoperative and neither draws nor supplies any energy sincethe potentials of the point A and B are identical.

FIG. 2 illustrates the invention in more detail giving a full circuitdiagram and further illustrates the invention for use with a three phasemotor. The three phase Motor 10 is connected by a plurality of connectora 440 volt three phase rnain power source. In a t yfical motor,the'conhe'ctbr Means 11, 12, and 13 will be connected to the main powersource via the Contacts 14, 15, and 16. For the heating unit of thepresent invention to operate with a three phase motor it is necessarythat one of the three Connecting Means 11, 12, and 13 not contain aswitch, e.g. Connecting Means 12 could be directly connected to one ofthe three phases without the switch 15, or a Jumper 17 can be providedto complete the connection of one phase (the phase) from the Motor tothe main power source.

The Timing Circuit 18 of FIG. 2 is illustrated as consisting of theCapacitors 19 and 20 and the Resistor 21 connected as shown. TheResistance 22 of FIG. 2 is shown as consisting of the Resistors 23 and24 connected in parallel to one another. The Gate Means 25 of FIG. 2 isshown as a back-to-back diode and more specifically as a solid stateBack-to-Back Diode 26. The Auxiliary Source of Electrical Energy 27 ofFIG. 2 is shown as a solid state power source 28. The Back e.m.f.Quenching Circuit 29 of FIG. 2 is shown as a Capacitor 30 connected inseries with a Resistor 31 and is connected across Switch 16.

OPERATION OF THE HEATING UNIT Referring to FIG. 1, when the Switch 4 isin the open position the heating unit will begin to draw current fromthe main power source. The Timing Circuit 5 will convert thesubstantially sinusoidal voltage signal of the main power source into asignal comprising only a portion of each sinusoidal cycle, e.g. 1 to ofeach half cycle. When the output from the Timing Circuit 5 is apredetermined positive or negative value, e.g. plus orminus 30 volts,the Gate Means 6 will be activated and will pass a triggering signal tothe Auxiliary Source'of Electrical Energy 8. Each time the AuxiliarySource of Electrical Energy 8 is activated by a triggering pulse fromthe Gate Means 6 it will supply a portion of a cycle of electricalcurrent. The shape and amplitude of the output of the timing circuit isdetermined by the values of the components forming the timing circuitand by the specific arrangement of the components used therein. Theportions of cycles of current from the Auxiliary Source of ElectricalEnergy 8 will then pass through the Motor 1 and keep the motor warm. Theparameters of the circuitry can be carefully adjusted to match thecharacteristics of the motors being heated so that a precise temperatureor temperature range can be maintained. More specifically, this isaccomplished by adjusting the values of the components of the timingcircuit.

When the Switch 4 is in the closed position the points A and B will beat the same potential and hence the heating unit, although connected tothe power source and one input of the motor, will not draw any currentand will not deliver any current to the motor. The Back e.m.f. QuenchingCircuit 9 will absorb any back e.m.f. induced in the armature of themotor when it is suddenly switched off. The Timing Circuit 5 providesinitially a slight delay in the application of electric current from theAuxiliary Source of Electrical Energy 8 to the Motor 1 in that thecapacitors included in the timing circuit take a finite time to becomecharged. Thus, after the motor is switched off it will have time to slowdown somewhat before the portions of cycles of current are appliedthereto from the Auxiliary Source of Electrical Energy 8.

PROCESS In FIG. 3 the output of the timing circuit of FIG. 2 is shownfor a line voltage of 110 volts.

FIG. 3 further illustrates the process of the invention. As illustratedin FIG. 3, the output of the timing circuit may attain a nonzero valueat zero radians (or elsewhere) and continue for a portion of a cycle,e.g. as illustrated to b radians, assume a zero output and then resume anonzero output, e.g., from 1r to d radians. From a to b radians and fromc to d radians the output of the timing circuit will be sufficientlyhigh, over 30 volts positive or negative as illustrated so that the gatemeans will be activated which will in turn lead to the triggering of theauxiliary source of electrical energy.

FIG. 4 illustrates the triggered operation of the auxiliary source ofelectrical energy. When said auxillary source is a full wave siliconswitch. When the gate means is activated, from a to b radians and from cto d radians, the auxiliary source of electrical energy sends portionsof cycles of current through the motor. The output of the auxiliarysource of electrical energy is illustrated for the sake of convenienceas being in phase with the line voltage although this will notnecessarily be the case during actual operation.

The process of the present invention is thus the application to thewindings of an electric motor not in operation of an alternating currentcomposed of chopped or partial portions of sinusoidally alternatingcurrent. Preferably, the series of partial portions of suppliedsinusoidally alternating current are supplied by solid state circuitry.Further, the portions of cycles of current are supplied by circuitrywhich is connected to the motor when the motor is in operation as wellas when the motor is off but which does not operate when the motor is inoperation.

The motor heater unit of the present invention, when solid statecircuitry is utilized, is considerably smaller and less expensive thanthe prior art motor heater units which supply a constant full 360sinusoidally altemating current of low voltage to the motor when it isshut off. Further, the motor heatihg unit of the present invention doesnot require the use of a time delay device to protect it from backe.m.f. induced in the armature of the motor when the motor is switchedoff. Still further, the heating unit of the present invention is alwaysconnected to the motor and does not require complicated switching meansto connect it thereto.

Any of a number of solid state components may be used as theBack-to-Back Diode 26 or the solid state power source 28. However, wehave found that for the Back-to-Back Diode 26 a General Electric ST-2 orRCA Type 40583 DIAC are especially useful and for the solid state powersource 28 a General Electric SC60e or RCA Type 2N544l TRIAC areespecially useful. These latter devices are gate-controlled, fullwavesilicon AC switches which switch whenever a positive or negative gatevoltage above a fixed value is introduced to the gate. Half wave siliconswitches may also be utilized. Thus, such switches can serve as a timecontrolled power supply if keyed to a sinusoidally varying voltage, asshown in FIGS. 3 and 4.

While specific and preferred embodiments of the invention have beendescribed herein, it will be understood that the invention is not meantto be limited thereto since variations therein will be apparent to thoseskilled in the art. The invention is intended to be limited thereforeonly by the scope and spirit of the appended claims.

That which is claimed is:

1. A motor heater unit for maintaining an AC electric motor in a warmcondition when the motor is not in operation, comprising, incombination:

a main power source;

a plurality of connector means connecting said main power source to saidmotor;

switch means in at least all but one of said plurality of connectormeans for turning said motor on and off;

a timing circuit adjusted to pass a portion of the wave form of anelectrical signal applied thereto, said timing circuit being connectedacross the first switch means;

a gate means connected to receive the output of said timing circuit;

an auxiliary power source connected across said first switch means andfurther connected to receive the output of said gate means, saidauxiliary power source providing electrical energy only when said gatemeans triggers said auxiliary power source; and

a back e.m.f. quenching means connected across said first switch means.

2. A motor heater unit as in claim 1 wherein said auxiliary power sourcereceives its electrical energy from the line voltage obtained from saidfirst switch means when said switch means is switched to open theconnection along the first connector means.

3. A motor heater unit as in claim 2, wherein said gate means comprisesa back-to-back diode.

4. A motor heater unit as in claim 3, wherein said back-to-back dioderequires at least a minimum positive or negative voltage to activate it.

5. A motor heater unit as in claim 4 wherein said auxiliary power sourceis a gate-controlled, full-wave or half wave silicon AC switch whosegate input is connected to the output of said gate means.

6. A motor heater unit as in claim 2, wherein the back e.m.f. quenchingcircuit comprises a resistor and a capacitor connected in series.

7. A motor heater unit for use in conjunction with a main power sourceto maintain an AC electric motor in a warm condition when said motor isnot in operation, comprising, in combination;

a plurality of connector means connecting said main power source to saidmotor;

switch means in at least all but one of said plurality of connectormeans for turning said motor on and ofi;

a timing circuit adaptable for passing a portion of the wave form of anelectrical signal applied thereto, said timing circuit being connectedacross the first of said switch means;

a gate means connected to receive the output of said timing circuit;

an auxiliary power source connected across said first switch means andfurther connected to receive the output of said gate means, saidauxiliary power source providing electrical energy only when said gatemeans triggers said auxiliary power source; and a back E.M.F. quenchingmeans connected across said first switch means.

1. A motor heater unit for maintaining an AC electric motor in a warmcondition when the motor is not in operation, comprising, incombination: a main power source; a plurality of connector meansconnecting said main power source to said motor; switch means in atleast all but one of said plurality of connector means for turning saidmotor on and off; a timing circuit adjusted to pass a portion of thewave form of an electrical signal applied thereto, said timing circuitbeing connected across the first switch means; a gate means connected toreceive the output of said timing circuit; an auxiliary power sourceconnected across said first switch means and further connected toreceive the output of said gate means, said auxiliary power sourceproviding electrical energy only when said gate means triggers saidauxiliary power source; and a back e.m.f. quenching means connectedacross said first switch means.
 2. A motor heater unit as in claim 1wherein said auxiliary power source receives its electrical energy fromthe line voltage obtained from said first switch means when said switchmeans is switched to open the connection along the first connectormeans.
 3. A motor heater unit as in claim 2, wherein said gate meanscomprises a back-to-back diode.
 4. A motor heater unit as in claim 3,wherein said back-to-back diode requires at least a minimum positive ornegative voltage to activate it.
 5. A motor heater unit as in claim 4wherein said auxiliary power source is a gate-controlled, full-wave orhalf wave silicon AC switch whose gate input is connected to the outputof said gate means.
 6. A motor heater unit as in claim 2, wherein theback e.m.f. quenching circuit comprises a resistor and a capacitorconnected in series.
 7. A motor heater unit for use in conjunction witha main power source to maintain an AC electric motor in a warm conditionwhen said motor is not in operation, comprising, in combination; aplurality of connector means connecting said main power source to saidmotor; switch means in at least all but one of said plurality ofconnector means for turning said motor on and off; a timing circuitadaptable for passing a portion of the wave form of an electrical signalapplied thereto, said timing circuit being connected across the first ofsaid switch means; a gate means connected to receive the output of saidtiming circuit; an auxiliary power source connected across said firstswitch means and further connected to receive the output of said gatemeans, said auxiliary power source providing electrical energy only whensaid gate means triggers said auxiliary power source; and a back E.M.F.quenching means connected across said first switch means.